Weather conditions in NSW are set to worsen, with rain currently lashing the mid north coast to move south to the Hunter, Sydney and Illawarra by the weekend.
Sydneysiders have been warned of "dangerously wet conditions" and potential flooding with the Hunter and Illawarra also issued warnings.
The Bureau of Meteorology says the focus on Thursday is for the mid north coast, where persistent and heavy rain is increasing the chance of flooding and flash flooding.
The weather bureau's Agata Imielska said the low pressure system was expected to move south to the Hunter, Sydney, and Illawarra regions on Friday and Saturday.
She says wet soils, combined with heavier falls, could result in dangerous conditions and flooding on the mid north coast.
While the bureau said the system was not expected to develop into an east coast low, it was still a "significant system".
Forecasters say more than 100mm of rain is expected to fall in a short period of time in some places.
"We've got heavy rainfall, we've got catchments which are very wet and ... we're very concerned about the potential for flooding along the coastal rivers from the mid north coast down to Sydney," said Justin Robinson, the bureau's flood operations manager.
"We've got widespread flooding, the focus today is on the mid north coast and we expect that rainfall to move south today and into the weekend and then we'll see that flooding develop in the Hunter and then further into the areas around Sydney."
There are several flood warnings in place on the state's rivers from the Orara River in the north, with the potential for minor flooding expected on the Georges, Nepean and Hawkesbury rivers in the Sydney region.
"As we move into the weekend there's a good potential for localised flash flooding across the Sydney metro area, and further south to the Illawarra," Mr Robinson said.
The State Emergency Service has warned motorists to avoid unnecessary travel this weekend.
The Bureau Of Meteorology has issued a severe weather warning for heavy rainfall for the mid north coast from Thursday.
Towns likely to be affected include Coffs Harbour, Port Macquarie, Taree, Woolgoolga, Sawtell, Bellingen and Dorrigo.
The inland of NSW is expected to get its share of the rain when a separate system moves in next week.